An in vitro tissue culture model using HeLa cells has been evaluated as a system to assess the pathogenic properties of Salmonella. The attachment of Salmonella with HeLa cells may be reduced by mannose and mannose analogs but the incidence of infection is not reduced. The implication of some other mechanism for invasion is currently being explored with low invasive mutants isolated from pathogenic strains. Bacteriophage isolated from Salmonella pathogens are related physiochemically to phage P22. Restriction enzyme mapping indicates specific differences in the restriction map. Lysogeny of Salmonella strains appears to contribute to the ability of a strin to invade the HeLa cell. Lysogeny does not effect the intracellular multiplication of the microbes. Electron microscopic studies of thin-section preparations of infected HeLa cells demonstrate the existence of Salmonella within endocytic vacuoles with numerous abnormal changes in the ultrastructure of the HeLa cells. Salmonella were shown to induce 3'-nucleotide phosphodiesterase in human T-lymphocytes, implying a cell-mediated immune response in Salmonellosis. Salmonella isolated from clinical specimens of animal origin carry numerous plasmids. The characterization of the plasmids, their role in perpetrating the disease syndrome in animals and transmission of this plasmid to other strains is in progress. The transmission of antimicrobial resistance to Escherichia coli HB 101 and Salmonella is low and only 2 to 5 plasmids have successfully transferred.